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FEL fires new life into old technology

Photonics SpectraDec 2012
SANTA BARBARA, Calif. – A high-power laser has amped up decades-old electron paramagnetic resonance (EPR) spectrometers to more efficiently study the world at the atomic level.

The multiuniversity team that enhanced the spectrometer has used it to study the electron spin of free radicals and nitrogen atoms trapped inside a diamond. The improvement pulls back the veil that shrouds the molecular world, allowing scientists to study tiny molecules at high resolutions, the investigators say.

EPR spectroscopy has existed for decades but has been limited by the electromagnetic radiation source it uses to excite electrons. Such electrons emit radiation that reveals details about the structure of targeted molecules. At EPR’s more powerful high magnetic fields and frequencies, pulses of power rather than continuous waves excite the targeted electrons.

Until recently, EPR spectroscopy was performed with a few tens of gigahertz of electromagnetic radiation. Now, using the free-electron laser (FEL) at the University of California, Santa Barbara – which emits a pulsed beam of the radiation – scientists from UCSB, the University of Southern California and Florida State University have powered an EPR spectrometer with 240 GHz of electromagnetic radiation.

Left, an electron paramagnetic resonance (EPR) spectrometer at the University of California, Santa Barbara. The device was used to study the electron spin of free radicals and nitrogen atoms trapped inside a diamond. Right, using UCSB’s free-electron laser, shown here, a multiuniversity team used 240 GHz of electromagnetic radiation to power an EPR spectrometer, yielding a more efficient tool for atomic-level studies.
“With FEL-powered EPR, we have shattered the electromagnetic bottleneck that EPR has faced, enabling electrons to report on faster motions occurring over longer distances than ever before,” said UCSB physics professor Mark Sherwin in a university release. The breakthrough could facilitate drug discoveries and more efficient polymer photovoltaics.

“In organic solar cells, light first creates charge carriers, and these must be collected by electrodes for the energy to be harvested,” Sherwin told Photonics Spectra. “The charge carriers, called polarons, often don’t make it out to the electrodes but get trapped in the material. Using FEL-powered EPR, we plan to study the charge generation and trapping processes in devices at room temperature. If we can identify the trapping mechanisms and sites, then materials scientists can work to eliminate them, leading to more efficient plastic solar cells.”

There is more to be done at 240 GHz, Sherwin said, but the team now has its sights set on 340 GHz.

“We are currently building a new FEL that will be optimized for pulsed EPR,” he said. “It will produce 100 times more power than the current FEL at 240 GHz, and pulse at 10 times the repetition rate with much greater stability. As far as the frequency, we are actually limited by the size of our magnet. Our existing 12.5-tesla magnet will enable us to reach 340 GHz.”

The team also plans to attack a major obstacle: reducing the EPR spectrometer’s “dead time,” or time between when the FEL pulse arrives at the sample and when the detector is turned on.

“We are also investigating methods to generate more complex pulse sequences in which both the phase and amplitude of the FEL pulses are controlled,” Sherwin said.

Also in the works is a new class of “spin labels” – small molecules that can be attached to specific sites on proteins to study structures and dynamics – optimized for use at the high magnetic fields and frequencies used by the researchers. The technology was developed with professors Song-I Han of UCSB, Daniella Goldfarb of Weizmann Institute of Science in Rehovot, Israel, and Adelheid Godt of the University of Bielefeld in Germany.

“These new spin labels take advantage of the remarkable magnetic properties of gadolinium ions, which are currently used in agents that enhance the contrast of magnetic resonance imaging,” Sherwin said. “With the new spin labels, we expect to be able to measure protein structure at physiological temperatures over significantly longer distances than is currently possible.”

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